Inactivated SARS-CoV-2 Variant Omicron, Lineage BA.2.3 (USA/MD-HP24556/2022)

On November 24, 2021, South Africa''s Minister of Health reported that the South African Genomic Surveillance Network had identified a new, rapidly spreading SARS-CoV-2 variant. Just two days later, the World Health Organization (WHO) declared this variant the fifth SARS-CoV-2 variant of concern (VOC) and named it Omicron (B.1.1.529). Although information on Omicron''s transmissibility and pathogenicity was very limited, its declaration as a VOC was much faster than that of the previous cases of Alpha (B.1.1.7), Beta (B.1.351), Gamma (P.1), and Delta (B.1.617.2) VOCs. The alarming discovery prompted swift action by the WHO due to the alarming number of viral mutations, particularly in the viral spike protein, which mediates infection of human cells and is the primary target of the humoral immune response.

Consistent with this, numerous recent studies have reported that Omicron VOCs are significantly less sensitive to neutralizing antibodies induced by prior SARS-CoV-2 infection or vaccination. However, some studies have shown that individuals boosted with mRNA vaccines elicit neutralizing humoral immune responses against Omicron VOCs that, while attenuated, remain sufficiently potent. Furthermore, most known T cell epitopes are conserved, suggesting that T cell responses in individuals previously infected and/or vaccinated will remain effective against Omicron VOCs. In a golden Syrian hamster model, a slight increase in transmissibility was observed with Omicron VOCs compared to Delta VOCs. Most individuals infected with Omicron in Denmark and Norway have reportedly been symptomatic. However, recent evidence suggests that Omicron infections result in more asymptomatic carriers, milder symptoms, and lower hospitalization and mortality rates compared to other SARS-CoV-2 VOC infections.